1. Field of the Invention
The present invention relates to an organometallic complex, and in particular, relates to an organometallic complex that is able to convert an excited triplet state into luminescence. In addition, the present invention relates to a light-emitting element that has an anode, a cathode, and a layer including an organic compound from which luminescence can be obtained by applying an electric field (hereinafter, referred to as “a layer including a luminescent material”).
2. Description of the Related Art
Organic compounds (organic molecules) are brought into a state with energy (excitation state) by absorbing light. By going through this excitation state, various reactions such as photochemical reactions are caused in some cases, or luminescence is produced in some cases. Therefore, the organic compounds have found various applications.
As one example of the photochemical reactions, a reaction (oxygen addition) of singlet oxygen with an unsaturated organic molecule is known (refer to Non-Patent Reference 1, for example). Since the ground state of an oxygen molecule is a triplet state, oxygen in a singlet state (singlet oxygen) is not generated by a direct photoexcitation. However, in the presence of another triplet-excited molecule, singlet oxygen is generated to achieve an oxygen addition reaction. In this case, a compound that is capable of forming the triplet excited molecule is referred to as a photosensitizer.
(Non-Patent Reference 1)
Haruo INOUE, et al., Basic Chemistry Course PHOTOCHEMISTRY I (Maruzen Co., Ltd.), pp. 106-110 (1999)
As described above, in order to generate singlet oxygen, a photosensitizer that is capable of forming a triplet excited molecule by photoexcitation is necessary. However, since the ground state of an ordinary organic compound is a singlet state, photoexcitation to an excited triplet state is a forbidden transition, and a triplet excited molecule is unlikely to be generated (a singlet-excited molecule is usually generated). Therefore, as such a photosensitizer, a compound in which intersystem crossing from the excited singlet state to the excited triplet state easily occurs or a compound in which the forbidden transition of photoexcitation directly to the excited triplet state is allowed is required. In other words, such a compound can be used as a photosensitizer, and is useful.
Also, such a compound often discharges phosphorescence. The phosphorescence is luminescence generated by transition between different energies in multiplicity and, in the case of an ordinary organic compound, indicates luminescence generated in returning from the excited triplet state to the singlet ground state (in contrast, luminescence in returning from an excited singlet state to a singlet ground state is referred to as fluorescence). Application fields of a compound that is capable of discharging the phosphorescence, that is, a compound that is capable of converting an excited triplet state into luminescence (hereinafter, referred to as a phosphorescent compound), include a light-emitting element using an organic compound as a luminescent compound.
This light-emitting element is a device attracting attention as a next-generation flat panel display element in terms of characteristics such as being thin and light in weight, high speed response, and direct current low voltage driving. In addition, because of being a self light emitting element and having a wide viewing angle, the light-emitting element has a comparatively favorable visibility, and is considered to be effective as an element to be used for a display screen of a mobile appliance.
In the case of using an organic compound as a light emitter, the emission mechanism of a light-emitting device is a carrier injection type. Namely, by applying a voltage with a light-emitting layer sandwiched between electrodes, an electron injected from a cathode and a hole injected from an anode are recombined in the light-emitting layer to form an exited molecule, and energy is released to emit light when the excited molecule returns to the ground state.
As the type of the excited molecule, as in the case of photoexcitation described above, an excited singlet state (S*) and an excited triplet state (T*) are possible. Further, the statistical generation ratio thereof in a light-emitting element is considered to be S*:T*=1:3 (refer to the following Non-Patent Reference 2, for example).
(Non-Patent Reference 2)
Tetsuo TSUTSUI, Textbook for the 3rd Workshop, Division of Molecular Electronics and Bioelectronics, Japan Society of Applied Physics, pp. 31-37 (1993)
However, in the case of ordinary organic compounds, luminescence (phosphorescence) from an excited triplet state is not observed at room temperature while only luminescence from an excited singlet state (fluorescence) is usually observed. This is because T*--->S0 transition (phosphorescence process) is a strong forbidden transition and S*--->S0 transition (fluorescence process) is an allowed transition since the ground state of an organic compound is usually a singlet ground state (S0).
Accordingly, the internal quantum efficiency (the ratio of photons generated to injected carriers) in a light-emitting element is assumed to have a theoretical limit of 25% in accordance with S*:T*=1:3.
However, since the T*--->S0 transition (phosphorescence process) is allowed when the phosphorescent compound described above is used, the internal quantum efficiency can be improved to 75 to 100% in theory. Namely, a luminous efficiency that is 3 to 4 times as much as that of the conventional one can be achieved. Actually, light-emitting elements using phosphorescent compounds have been reported one after another, and the luminous efficiencies thereof have been attracting attention (refer to Non-Patent References 3 and 4, for example).
(Non-Patent Reference 3)
D. F. O'Brien, et al., Applied Physics Letters, vol. 74, No. 3, pp. 442-444 (1999)
(Non-Patent Reference 4)
Tetsuo TSUTSUI, et al., Japanese Journal of Applied Physics, vol. 38, pp. L1502-L1504 (1999)
A porphyrin complex that has platinum as a central metal and an organometallic complex that has iridium as a central metal are used respectively in Non-Patent References 3 and 4, which are both phosphorescent compounds.
Further, by alternately stacking a layer including an organometallic complex that has iridium as a central metal (hereinafter, referred to as an iridium complex) and a layer including DCM2 that is a known fluorescent compound, it is possible to move triplet excitation energy generated in the iridium complex to DCM2 so as to contribute to luminescence from DCM2 (refer to Non-Patent Reference 5, for example). In this case, since the volume of the excited singlet state of DCM2 (usually 25% or less) is amplified more than usual, the luminous efficiency of DCM2 is increased. This can be said to be, in a manner, a sensitization effect of the iridium complex that is a phosphorescent compound.
(Non-Patent Reference 5)
M. A. Baldo, et al., Nature (London), vol. 403, pp. 750-753 (2000)
As described in Non-Patent References 3 to 5, a light-emitting element using a phosphorescent compound is capable of achieving a higher luminous efficiency than conventional ones (that is, capable of achieving a higher luminance with a small current). Therefore, it is considered that a light-emitting element using a phosphorescent compound will play a great role in future development as a method for achieving luminescence with a higher luminance and a higher luminous efficiency.
As described above, in phosphorescent compounds, intersystem crossing is likely to occur, and luminescence (phosphorescence) from an excited triplet state is unlikely to be produced. Therefore, phosphorescent compounds are useful in using as a photosensitizer and applying to a light-emitting element as a phosphorescent material, and expected compounds. However, the number thereof is small under the present situation.
Among the small number of phosphorescent compounds, the iridium complex used in Non-Patent Reference 4 or 5 is one kind of organometallic complexes, referred to as orthometalated complexes. Since this complex has a phosphorescence life of several hundreds of nanoseconds and is high in phosphorescence quantum yield, the reduction in efficiency due to increase in luminance is smaller than that of the porphyrin complex described above. Therefore, the complex is effectively used in a light-emitting element. For this reason, such an organometallic complex is one of guidelines for synthesizing a compound in which direct photoexcitation or intersystem crossing to the triplet excited state is likely to occur, and furthermore, a phosphorescent compound.
The iridium complex used in Non-Patent References 4 or 5 has a relatively simple ligand structure and shows green luminescence with favorable color purity. However, it is necessary to change the ligand structure in order to change the luminescent color to a different one. For example, in Non-Patent Reference 6, various ligands and iridium complexes using the ligands are synthesized, and several luminescent colors are realized.
(Non-Patent Reference 6)
Mark E. Thompson, et al., The 10th International workshop on Inorganic and Organic Electroluminescence (EL'00), pp. 35-38 (2000)
However, since these are very limited examples, and insufficient satisfaction is obtained as for the kinds thereof. The organometallic complexes described above are materials in which intersystem crossing is likely to occur, and versatile application such as a photosensitizer and a phosphorescent material can be considered. Therefore, versatile performance is required depending on the application.
Accordingly, novel organometallic complexes in which intersystem crossing to an excited triplet state is likely to occur are desired. Above all, a novel organometallic complex that can be used as a phosphorescent material is desired in order to obtain a particularly high-efficiency light-emitting element.